4 
20 THE OCEAN WORLD. 
deposited, accumulating in the large depressions of the soil. The 
seas of the primitive globe were thus formed of rain water, holding 
insolution all that the earth had given up, collected in large basins. 
Chloride of sodium, sulphates of soda, magnesia, potassium, lime, 
and silex, this latter in the form of a soluble silicate ; in a word, 
every soluble matter that the primitive globe contained formed part 
of the mineral contingent of this water. If we reflect that through 
all time up to the present day none of the general laws of Nature 
have changed—if we consider that the soluble substances contained 
in the water of the primitive seas have remained there, and that the 
fresh water of the rivers constantly replaces the water which disap- 
pears by evaporation—we have the true explanation of the saltness 
of seawater. “Itis avery simple theory, it is true,” adds M. Figuier, 
“but one that we have found nowhere, and the responsibility of 
which we therefore claim. The chloride of sodium is by no means 
the only substance dissolved in sea water. It contains, besides, 
many other mineral substances ; in short, every soluble salt on the 
face of the globe, and, along with them, portions of different metals 
in infinitely small quantities.” 
The mean temperature of the surface of the sea is nearly the 
same as the atmosphere, so long as no currents of heat or cold in- 
terpose their perturbing influence. In the neighbourhood of the 
Tropics, it appears that the surface of the water is slightly warmer 
than the ambient air, but experiments on the temperature of the sea 
from the surface to the bottom reveal, according to our author,* 
“some evidence which establishes a curious law. In very deep 
water a perfectly uniform temperature of 4° below zero prevails, 
which corresponds, as physics have established, to the maximum 
density of water. Under the Equator this temperature exists at 
the depth of 7,000 feet. In the Polar regions, where water is 
colder at the surface, this temperature is maintained at 4,600 feet. 
The isothermal lines of 4° form a line of demarcation between the 
zones, where the surface of the sea is colder, and those where it is 
warmer than the bed of four degrees below zero.” This is more 
clearly shown in Fig. 4, which represents a section of the ocean, the 
curved line which touches two points at the surface indicating the 
depths where the temperature is constantly fixed at 4°. 
Dr. Maury’s account of this phenomenon is asserted with less 
confidence. ‘The existence of an ésothermal floor of the ocean, as he 
calls it, was first suggested by the observations of Kotzebue, Admiral 
* “La Terre et les Mers,” p. 517, troisiéme édition. 
